1. Technical Field
The present invention relates in general to ceramic armor materials and, in particular, to an improved system, method, and apparatus for improving the performance of ceramic armor materials with shape memory alloys that retain the ceramic in a state of compression.
2. Description of the Related Art
In the prior art, there are numerous types of ballistic armor used to defend targets. Metals and metallic alloys are the most common materials used to fabricate armor, but other materials such as plastics, woven materials, and ceramics also have been used. Multi-layered armors formed from dissimilar materials (e.g., a ceramic strike plate on a metallic base) are also known and suitable for some applications.
Ceramic materials are very strong in compression, but weak in tension. They are also very brittle, but can have significant strength after fracture when under compression. They also tend to be lightweight when compared to other materials such as metals. These characteristics make ceramics well suited for armor applications, but also make them very complex and difficult to understand.
When ceramic armor is impacted by a projectile, one of its primary failure mechanisms is through propagation of an acoustic wave to the back surface of the ceramic strike plate. The acoustic wave reflects off the interface and puts the back face of the ceramic material in tension. As described above, ceramic materials respond poorly to tensile loads such that a ceramic strike plate fails due to cracking that originates at the back face of the strike plate.
One solution to this problem puts the back face of the ceramic strike plate in residual compression in order to increase the amount of load that the strike-plate can withstand before failure begins. For example, the coefficient of thermal expansion (CTE) mismatch between the ceramic and metallic materials may be used advantageously in this manner. Since metals thermally expand much more readily than ceramic materials, the entire armor system may be heated to elevated temperature (e.g., >500° C.) such that the dissimilar materials are bonded together at the elevated temperature before being cooled to form the bonded product. Upon cooling, the metal shrinks more than the ceramic but is constrained by the bond between them so that the ceramic receives residual compressive stresses at its interfacing surface with the metal. Unfortunately, the amount of strain recoverable (approximately 0.3%) also is limited by thermal expansion/contraction considerations. In addition, this method requires difficult assembly procedures in high temperature furnaces with complex tooling requirements. Thus, an improved solution for joining dissimilar materials for ballistic armor applications would be desirable.